Electromagnetic clutches are well established in the art. U.S. Pat. No. 1,880,061 was issued in 1932 for the first electromagnetic clutch. While they are best known for their use on automotive air-conditioning compressors, they have found a number of uses. They are less expensive than other types of clutches and are commercially available from a number of sources in various holding torque capacities. Their simplicity of operation leads to reliability and control without external actuators.
Despite these advantages, there are still a number of areas where they have not come into general use. One limitation has been the difficulty encountered in situations involving internal combustion engines driving high inertia loads. An example of such an application is use on an oil well pumping unit.
There are several reasons that electromagnetic clutches have problems with high inertia loads. First, it is normal engineering practice to select a clutch rated at a somewhat higher peak holding torque than the expected load so that there is no inadvertent slippage and wear in its operation. Secondly, it typically takes about two-thirds of the rated voltage to reliably engage the clutch, but only a very low voltage to hold the friction plates together after they have made contact. This is because the ferromagnetic material in the plates has a far higher magnetic permeability than the air gap separating them and can convey much greater magnetic force at a given current level. A third consideration is that, like any electromagnetic device, an electric clutch has inductance. This means that the rising magnetic field tends to resist the flow of current for a period of time after the voltage is applied. Conversely, the falling magnetic field tends to induce current after the voltage is removed. Since the holding torque of the clutch is proportional to the amount of current flowing, this results in a delay factor in the establishment or removal of holding torque. Typically, it takes about 100 milliseconds for holding torque to rise from zero to two-thirds of its maximum upon the application of voltage with commercially available electromagnetic clutches. A similar delay occurs when voltage is removed for the fall of holding torque. The combined result of these factors is that electromagnetic clutches tend to engage at high holding torques and a delay is built in before any action to reduce holding torque can be effective.
The manner in which the engine power is controlled can also be a contributing factor to this problem. A common means of controlling an engine is a mechanical governor which uses springs and flyweights to position a throttle to maintain a preset speed. This arrangement has the engine set at full throttle prior to the engine being started. Often a device such as a solenoid is employed to pull the throttle back, against the force of the governor springs, to a low speed idle position for starting and warmup. If such an idle control device is present, it is released prior to attempting to put the engine under load. After warmup, with or without an idle controller, the engine is then allowed to run at the governed speed at a low power setting, or governed idle speed, since no load has yet been applied. Another means of controlling the engine is an electronic governor, as in U.S. Pat. No. 5,003,948. The electronic governor typically calculates speed by measuring the time between pulses from one of several known types of electronic sensors and adjusts the throttle position with an actuator to maintain a preset speed. Solenoids and stepper motors are often used as throttle actuators and a variety of methods are known for mounting them on engines and linking them to throttles. Also, electronic governors are used to control fuel injectors on engines so equipped. Engines controlled by any of these means are referred to as speed-governed engines.
There is a typical sequence of events that occurs when an electromagnetic clutch is used on an internal combustion engine driving a high inertia load. First the engine is started, warmed up, and run at governed speed. Then voltage is applied to the clutch. The clutch engages, after a delay of about 100 milliseconds for the magnetic field to build and about another 20 milliseconds for the friction plates to move together, at a high holding torque. This holding torque is far greater than the torque being developed by the engine at governed idle speed. The inertia of the load then causes the engine to decelerate very rapidly. The governor opens the throttle or activates the fuel supply system very rapidly often causing instability in the fuel mixture, particularly in carbureted engines. The engine further slows down to a speed well below its optimum torque producing speed and is overwhelmed by the load. In this scenario, the engine never reaches its maximum torque producing capability and the clutch provides very limited slipping action to help start the load in motion.